The conversion of computer video signals to video signals compatible with today's televisions has become an important technology for business presentations, home entertainment and the network personal computer (NC). There are three major elements involved in this transformation, color space conversion, scan rate conversion, and encoding the composite waveform in accordance with a selected television signal format (i.e., either NTSC or PAL).
Color space conversion transforms the RGB (Red, Green, Blue) signals output by a VGA source to the YUV (Luminance and Chrominance) signals used to create composite video. This can be performed before or after the scan rate conversion, but must be performed before encoding to a composite television signal.
Encoding to a composite television waveform involves modulation of the color difference signals, generation of synchronization signals, bandlimiting of the luminance and chrominance signals and summation of luminance, chrominance and sync signals.
Scan rate conversion produces a sequence of interlaced lines of video from the non-interlaced sequence of lines generated by the VGA source. In video displays, such as a television or a VGA display the picture is created by scanning the electron bean horizontally across the screen from left to right, the moving back to the left, and scanning across the screen again. This process is repeated until all lines have been scanned, thus completing one frame of video. The beam also moves down the screen until it reaches the bottom of the display, at which point the beam returns to the top. Referring to FIGS. 1-5, general aspects of the scan rate conversion process are now described.
FIG. 1 shows a non-interlaced scan pattern typical of VGA displays. Because scan rast converters convert VGA signals 10 television signals, the scan pattern of FIG. 1 illustrates the pattern of VGA lines presented to the scan rate converter. In a non-interlaced VGA display every line is scanned in every frame. At the completion of one scan line, the electron beam travels back to the left side of 10 screen before the next line can be displayed. This is called retrace. A popular VGA display frame rate (i.e., the rate at which new frames are displayed) is .about.60 Hz.
FIGS. 2 and 3 show an interlaced scan pattern typical of a television display. On a television display every other line is scanned in what is called the "first field" (FIG. 2), and the alternate lines are displayed in the "second field" (FIG. 3). Both fields are required to make a complete picture. For NTSC signals, the field rate (i.e., the rate at which new fields are displayed) is .about.60 Hz and the frame rate .about.30 Hz.
Scan rate conversion must account for the different frame rates of the VGA monitors and television monitors. For example, since a television monitor displays half as many lines as a VGA monitor in a given amount of time, each line of the television signal generated by a scan rate converter must have a duration that is twice that of the VGA signal. Conventional scan rate converters account for the different frame rates by writing the VGA data to memory at one rate and reading television data out of memory at one half of this rate.
In addition to extending the duration of each line, conventional scan rate converters perform a filtering function on adjacent VGA lines. The reason for filtering is to reduce flicker in the television picture caused by picture content transitions (i.e., image edges) in the vertical direction in the VGA signal. For example, if only half of the VGA lines were to appear in a television field, these transitions would appear to move up and down by one line at the frequency of the television frame rate (30 Hz). The flicker filter reduces this movement by spreading a transition over a sequence-of television lines. For example, one common flicker filter produces a television line by adding one quarter of the current VGA line, two quarters of the previous VGA line and one quarter of the VGA line before that. This is called a "1-2-1" or a "1/4-1/2-1/4"" filter. This filtering process increases the memory requirement in the scan rate conversion block. Typically, between one and one half and three VGA lines of memory are required per video component.
FIGS. 4 and 5 illustrate how three VGA lines (FIG. 4) are combined with a 1-2-1 filter to produce a corresponding television line (FIG. 5). The solid lines shown in FIG. 4 are the VGA lines being filtered. The line 41 (FIG. 4) is the current VGA line, corresponding to the television line 51 (FIG. 5) being output. The darker line 42 is the previous VGA line, which is multiplied by the largest weight (i.e., "2"). Both lines 41 and 43 are multiplied by the smallest weight (i.e., "1").
The above-described process of scan rate conversion does not change the location of the image content with respect to the beginning and ending of the electron beam scan in either the horizontal or vertical direction. However, unlike a VGA monitor, a television monitor overscans the picture tube, leaving some of the original image outside of the viewable screen. For example, FIG. 6 shows an image displayed on a VGA monitor and FIG. 7 shows the same image when displayed on a television monitor. Note that a significant portion of the image is not displayed on the television monitor. The amount of this overscan is typically on the order of five to ten percent. With traditional viewing material, where the original source was unknown to the viewer, this is not noticeable. However, when the television is displaying a converted computer display, the missing portions of the screen are noticeable, and sometimes important, areas such as menus.
This problem is addressed by scan rate converters that scale VGA images so that, when displayed on a television monitor, the VGA images fit fully within the television monitors viewable area. Conventional scaling scan rate converters (such as the Yuan Scan Rate Converter, Model: SFN-100) require the use of a frame buffer/memory large enough to capture an entire graphics frame. After a frame is captured, these conventional converters perform scaling scan rate conversion. This technique requires either a large part count (for implementation using discrete components) and corresponding printed circuit board space, or large amounts of on-chip memory (for monolithic integrated circuit implementation). This leads to high system cost and imposes serious limitations on the potential of high volume applications using conventional scaling scan rate conversion techniques.